My invention relates to a voltage controlled oscillator for a phase locked loop, and particularly to such a voltage controlled oscillator that has improved modulation sensitivity over the output radio frequency range of the voltage controlled oscillator.
In a phase locked loop, particularly in a frequency synthesizer, a control signal from the loop phase detector is applied to the loop voltage controlled oscillator to determine or set the output center frequency or the output radio frequency of the voltage controlled oscillator. This control signal is applied to a control varactor (or voltage variable capacitor diode) coupled to an inductor in a resonant circuit. This control signal changes the varactor capacity, and hence changes the output center frequency of the voltage controlled oscillator. At the same time, a modulation signal (such as a voice or data signal) is applied to a modulation varactor (or sometimes to the control varactor) to change the varactor capacity and hence vary the frequency of the voltage controlled oscillator about its output center frequency. Where the output center frequency of the loop voltage controlled oscillator must be switched over a relatively wide range (such as a number of radio frequency channels), the change in capacity of the control varactor is or must be similarly wide. To provide this change, the control varactor capacity must be switched between a low value at the high frequency end of the range and a high value at the low frequency end of the range. But the capacity change of the modulation varactor with modulation is relatively small over the entire range. If the ratio of the capacity change produced by the modulation varactor to the total capacity of the control varactor at the high frequency end of the range is 0.1 for example, the corresponding ratio at the low frequency end of the range might be only 0.01. Thus, the modulation sensitivity of the voltage controlled oscillator varies. Under these conditions, if the modulation sensitivity is set at a desired level at the high frequency end of the range, insufficient modulation will likely result at the low frequency end of the range, and information may be lost. Or, if the modulation sensitivity is set to a desired value at the low frequency end of the range, excessive or illegel frequency deviations may result at the high frequency end of the range.